According to her friend, Monique Rivalland, "...As we turned into Piazza IV Novembre we saw a vigil taking place. The stone steps in the square were carpeted with flickering candles and pictures of Meredith and the sounds of revelry had been replaced by a sombre silence. We left in shock. We were involved in the tragedy, at least emotionally. It felt bizarre to see so many strangers mourning the loss of our friend."
ARTICLE LINK
As far as I know Amanda has never said why she did not attend the vigil. I certainly have not read any source stating it was because she was too tired. Please let me know where you are getting that information from so I can read it.
Monique clearly did not even know there was a vigil until she accidentally stumbled across it. Perhaps it was not advertised very well. According to her, she felt it was too bizarre to see the many strangers mourning Meredith. Perhaps Amanda felt similarly.